Abscisic acid (“(S)-(+)-abscisic acid” or “S-ABA”) is a naturally occurring plant hormone which is found in all higher plants. (S)-(+)-abscisic acid is involved in many major processes during plant growth and development including dormancy, germination, bud break, flowering, fruit set, general growth and development, stress tolerance, ripening, maturation, organ abscission, and senescence. (S)-(+)-abscisic acid also plays an important role in plant tolerance to environmental stresses, such as drought, cold, and excessive salinity.
The naturally occurring enantiomeric form of abscisic acid is (S)-(+)-abscisic acid. In some literature reports the other enantiomer, (R)-(−)-abscisic acid is seen to be biologically inactive. In other research, it has been reported that (R)-(−)-abscisic acid also has some biological activities, however, they are often different from those of the (S)-(+)-enantiomer. See, Zeevart, J. A. D. and Creelman, R. A. (1988) Metabolism and Physiology of Abscisic Acid, Annu. Rev. Plant Physiol. Plant Mol. Biol. 39, 439-473. Thus, for use in a commercial agricultural product, the formulations of the present invention comprising (S)-(+)-abscisic acid as the active ingredient are preferable to the prior art formulations comprising racemic (R,S)-(±)-abscisic acid, because, in the best case for these prior art formulations, half of the racemic (R,S)-(±)-abscisic acid is inert, resulting in the need to purchase, formulate, package, ship, and apply twice as much material. In the worst case, the (R)-(−)-enantiomer in racemic (R,S)-(±)-abscisic acid can add undesirable side effects to the desired result given by the applied (S)-(+)-abscisic acid and potentially result in undesirable residue in food crops and in the environment.
The stereochemistry of the side chain of naturally occurring abscisic acid produced biosynthetically by all green plants and some microorganisms is 2-cis-,4-trans. The (S)-(+)-2-trans-,4-trans-isomer of abscisic acid also occurs naturally, being produced photolytically by the action of sunlight on the (S)-(+)-2-cis-,4-trans-isomer. The (S)-(+)-2-trans-,4-trans-isomer is reported to be biologically inactive. See, P. E. Kreidelmann, et al., Plant Physiol. 49, 842-847 (1972), D. P. Zhang, et al., Plant Physiol. 128, 714-725, (2002) or X. C. Yu, et al., Plant Physiol. 140, 558-579 (2006).
A challenge with agricultural formulations of (S)-(+)-abscisic acid salts is the inability to maintain color stability. Previously, the color of aqueous formulations of (S)-(+)-abscisic acid changed during storage from a near colorless solution to a dark yellow or brown solution. Such a result is undesirable because the formulation can have an inconsistent cosmetic appearance which raises questions regarding formulation quality and efficacy and can lower the product's commercial appeal. Therefore, there is an unmet need in the art for (S)-(+)-abscisic acid salt formulations with color stability.
Sodium citrate and sodium acetate are both listed in the Food and Drug Administration's (“FDA”) Select Committee on Generally Regarded as Safe Substances (“SCOGS”) Database, and therefore, are desirable alternatives to color stabilizers utilized by the prior art that are undesirable or prohibited in pesticide formulations used on food crops in the United States.
U.S. Pat. No. 8,722,928 is directed to aqueous formulations containing salts of (S)-(+)-abscisic acid, the acrylic copolymer surfactant Atlox™ 4913, and the color stabilizers sodium citrate and sodium acetate. The formulations of U.S. Pat. No. 8,722,928 have impressive color stability and are non-precipitating. While the formulations of U.S. Pat. No. 8,722,928 have desirable and commercially important qualities, they are not suitable for organic farming.
The Organic Materials Review Institute (“OMRI”) reviews agricultural products against organic standards to certify that products are appropriate for use in organic farming. Some of the organic standards include that the ingredients of the products are not obtained from genetically modified organisms, that all of the inert ingredients are included in the EPA List 4 §205.601(m), and that the natural products are extracted by methods that meet the 7 C.F.R. §205 criteria.
One of the advantages of the formulations of U.S. Pat. No. 8,722,928 is that they can be tank-mixed with the plant growth regulator ethephon without the formation of precipitates. This attribute, however, is not important to organic farmers because ethephon cannot be used in organic farming.
Therefore, there is a need for environmentally safe, non-phytotoxic, efficacious, ABA solution formulations for use in organic farming. The improved formulations should overcome the color stability, handling, storage, transportation, and solubility issues encountered by prior art formulations. The formulations should also be certified by OMRI to be safe for organic farming and be safe for end users.